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<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
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    onload=function () {
      let elementsByTagName = document.getElementsByTagName("tr");
      for (let i = 1; i < elementsByTagName.length; i++) {
        if(i%2==0) {
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<body>
    <table id="tab1" border="1" width="800" align="center" >

      <tr style="background-color: #999999;">
        <th>分类ID</th>
        <th>分类名称</th>
        <th>分类描述</th>
        <th>操作</th>
      </tr>
      <tr>
        <td>1</td>
        <td>手机数码</td>
        <td>手机数码类商品</td>
        <td><a href="">修改</a>|<a href="">删除</a></td>
      </tr>
      <tr>
        <td>2</td>
        <td>电脑办公</td>
        <td>电脑办公类商品</td>
        <td><a href="">修改</a>|<a href="">删除</a></td>
      </tr>
      <tr>
        <td>3</td>
        <td>鞋靴箱包</td>
        <td>鞋靴箱包类商品</td>
        <td><a href="">修改</a>|<a href="">删除</a></td>
      </tr>
      <tr>
        <td>4</td>
        <td>家居饰品</td>
        <td>家居饰品类商品</td>
        <td><a href="">修改</a>|<a href="">删除</a></td>
      </tr>
    </table>
</body>
</html>